1. Field of the Invention
This invention relates to a method and apparatus for supervising the combustion state inside a combustion furnace such as a boiler for thermal power generation.
Supervision of NO.sub.x formed during combustion of dust coal or CWM (coal water mixed), or unburnt components in ash is extremely important in order to keep a satisfactory combustion state. This is also extremely significant for the effective utilization of fuels. Efficient combustion and minimization of deterimental components in the exhaust smoke are important factors that determine a good combustion state.
The present invention relates to supervision technique in this field of combustion.
2. Description of the Prior Art
As a new lock has been taken at coal as an alternative energy source to petroleum, combustion technique of dust coal has drawn increasing attention. Although the combustion technique of dust coal itself is said to have reached completion, novel technique has become necessary in recent years to cope with severe official restrictions on the exhaust of air-polluting matters.
A dust coal fuel has a greater N content than liquid fuels such as heavy oil, naphtha, and the like, and hence generates a higher concentration of nitrogen oxides (hereinafter referred to as "NOx" that result in the air pollution upon combustion. Furthermore, fuel NOx that is formed due to the combination of the nitrogen molecules in the fuel with the oxygen molecules in the air for combustion does not much depend upon the combustion temperature in comparison with thermal NOx formed due to the dissociation and combination of the nitrogen and oxygen molecules in the air for combustion, or with prompt NOx formed due to the combination of hydrocarbons in the fuel with the oxygen molecules in the air for combustion. From this aspect, means for reducing the resulting NOx to N.sub.2 and the like is believed rather necessary than a combustion method which does not generate NOx, in order to reduce NOx of dust coal combustion. Since the dust coal fuel has many factors relating to its properties such as a fuel ratio, an ash content, a viscosity, a particle size distribution, and so forth, remarkable fluctuation occurs in the combustion process. Changes with time such as in pulverization, transportation, jetting by a burner, and the like, can not be neglected in comparison with combustion equipment for heavy oil, naphtha, LNG, and the like.
As described above, a combustion method which takes into consideration (i) a reducing effect of NOx and (ii) the changes of combustion must be developed in order to realize low NOx combustion of dust coal.
The increase of unburnt components in the ash reduces a boiler efficiency and imposes various limitations on the processing of waste. When high fuel ratio coal (solid carbon/volatile components) and low grade coal are used, means for reducing the unburnt components in the ash must be developed.
On the other hand, the combustion process of dust coal particles is such that decomposition and combustion of volatile components proceed at the initial stage of combustion, and then the surface combustion of coke-like residual carbonaceous matters (hereinafter referred to as "char") proceeds. The surface combustion of the char is somewhat slower than the decomposition combustion of the volatile components, and the most of the time required for complete combustion are believed to be used for the surface combustion of the char.
Therefore, it is extremely difficult to estimate the unburnt components in the ash during the combustion process because of a large number of factors relating to the properties of the dust coal such as the fuel ratio, the ash content, the viscosity, the particle diameter distribution, and so forth.
However, it is empirically obvious that combustion be made immediately inside a furnace in the presence of excessive oxygen (O.sub.2) and in a high temperature atmosphere in order to reduce the unburnt components in the ash, but such an operation method involves the problems of control and safety.
In existing business or industrial dust coal-fired boilers, the boiler operation is carried out in such a manner as to minimize the unburnt components in the ash to improve the boiler efficiency, but if a two-stage combustion method or a slow combustion method effective for gas- and oil-fired boilers is employed, the temperature inside the furnace tends to drop and the unburnt components in the ash tend to increase, on the contrary.
A supervision method which supervises the flame at the time of combustion using an ITV (industrial television) mounted to the opposed wall of a burner or a method inspecting the combustion state from a peep hole formed on a furnace wall has been employed in the past to determine the combustion state inside the furnace. However, either method merely supervises the combustion flame alone.
An automated supervision method uses a flame detector, but this method merely supervises ignition or extinguishment. In other words, this method is not a determined combustion supervision method, and inevitably relies upon the experience and skill of a furnace operator.
One of the conventional supervision methods is disclosed in U.S. Pat. No. 3,842,391 (entitled "Method and apparatus for flame monitoring", July 16, 1974). This prior art relates to a method and apparatus for monitoring the flame of a furnace using a plurality of burners such as a thermal generation boiler. This method uses two photosensors to optically monitor the flame of a selected burner from a plurality of burners. A signal having an a.c. component corresponding to the change of intensity of radiation from the flame is detected by the two sensors, and the degree of correlation is determined.
U.S. Pat. No. 4,403,941 (entitled "Combustion process for reducing nitrogen oxides", Sept. 13, 1983) discloses denitrating by combustion, but does not at all teach the supervision of the combustion state using flame-relating data. The reference describes only the reduction of NOx by multi-stage combustion.
U.S. patent application Ser. No. 527,847 (Aug. 30, 1983) filed previously by the Applicant of the present invention specifically deals with the shape of the flame. This prior application contemplates to diagnose the combustion state from parameters representing the shape of the root portion of the flame. The correlation between the shape of the flame root and the combustion state is stored in advance, and judgement is then made to which pattern the measured flame shape belongs, thereby diagnozing the combustion state. The present invention provides a combustion state supervising system as a result of a further improvement in this prior invention.
As to the physico-chemical behavior of combustion of dust coal, mention can be made of W. R. Seeker et al., "The Thermal Decomposition of Pulverized Coal Particles" (Eighteenth Symposium-International-On Combustion, The Combustion Institute, 1981, pp. 1213-1226).